The daughter of former British ambassador Sir Michael Weir, she is best known for her roles in The Fast Show and Posh Nosh, and for writing several books including the international best seller Does My Bum Look Big in This? (a catchphrase of one of her characters in The Fast Show). She is also well known for her books Onwards and Upwards and Cupid for which she also wrote the screenplay now owned by Working Title Films. Weir has written for The Independent magazine, which is included with the Saturday edition and The Guardian newspaper and Weekend magazine. She continues to contribute regularly to both newspapers.
She is married to Dr Jeremy Norton; the couple have two children. The actor David Tennant is a close friend and is godfather to her youngest child. When Tennant first moved to London in the early 1990s, he lodged with Weir at her house in Crouch End for five years; they had met on the set of the BBC TV series Takin' Over the Asylum.